Forty-four percent of likely voters said they would choose Brown in a hypothetical matchup against Warren, the former Wall Street bailout and consumer financial protection watchdog in the Obama administration, according to a WBUR poll released Tuesday.

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Thirty-five percent of Massachusetts voters said they would elect Warren, who stepped down from her position in the administration and launched an exploratory committee aimed toward supporting a challenge to Brown.

Eighteen percent of voters said they either didn't know or were undecided.

Warren is the Democratic candidate who's most competitive against Brown at this point in the race. Brown leads City Year founder Alan Khazei (D) by 15 points, activist Bob Massie (D) by 16 points, and Newton Mayor Setti Warren (D) by 18 percent.

Fifty-four percent of Bay Staters have a favorable impression of Brown, while 25 percent have an unfavorable view of the Republican senator.

Warren also enjoys a net favorable rating (17 to 13 percent), but a larger number of voters said they had either never heard of her, or had heard of her but hadn't made up their mind about her.

The poll, conducted Aug. 30 through Sept. 1, has a 4.4 percent margin of error.